Multiple output switching power supplies are known in which an unregulated d-c voltage (typically obtained from a rectified a-c voltage source) is periodically applied through switches to the primary winding of a transformer so as to induce voltage pulses in a plurality of secondary windings. The induced voltage pulses charge, through diodes, capacitors connected to the output of the secondary windings. The capacitors act as buffer filters, and relatively constant d-c voltages are available at their terminals. These voltages may be used to power loads.
By comparing the voltage available at the terminals of one of such filter capacitors with a reference voltage, an error signal is obtained which is used to control, through switchable devices, the frequency and/or the duty cycle of the voltage and current pulses applied to the primary winding in order to regulate the voltage at the terminals of one capacitor, thus of one output. Absent any independent control circuit for the other outputs, the charge voltage of the other capacitors, and hence the voltage available at the other outputs, is indirectly regulated and is largely affected by the load conditions of the several outputs in both static and transient situations.
To overcome this disadvantage, several solutions are known. They will be considered in detail in the following, but, in any event, they are not completely satisfactory, particularly in dynamic conditions of load variation. One of these known solutions provides for a series connection among outputs and requires the use of components sized to sustain the sum of the loads connected to the outputs. In addition the voltage ripple which affects the indirectly regulated output is the sum of the intrinsic ripple voltages and the one affecting the regulated output. Further in case of transient load peaks at the regulated output, a relevant voltage drop occurs at the indirectly regulated output.